The present invention relates to apparatus for locating the position of faults in power lines and in particular to a system which monitors open loop underground residential distribution (URD) power cable systems for fault signatures and uses the fault signatures to locate the faults.
Typical underground residential distribution power cable systems are configured as a loop with an open point along it. The URD loop cable sections are installed in between transformers that supply power to the individual customers. The underground cable system is fed from both ends by overhead power lines. Along the loop an open point is established so that each half is fed independently. When a fault occurs and the faulted section of cable is identified, the open point is moved to another transformer enclosure. In this manner the cable system can be sectionalized to isolate the failed cable section and quickly return power to the customer without having to repair the cable.
Sections of cable fail on a regular basis causing a blackout in the affected area. To return power to the customers, the repair crew must find the failed section of cable and reconnect the transformers with an available good section of cable. This process is called sectionalizing. To save time power utilities have installed devices called faulted circuit indicators (FCIs). In underground distribution systems, one device is installed on the cable at each transformer enclosure. The FCIs function as follows. Power is supplied to the cable from one end only (feed point); the other end is left open (open point). When the cable fails, a large over current passes through the cable from the feed point into the fault. This large over current trips the FCIs located in each transformer enclosure between the feed point and the fault. The FCIs in the transformer enclosures between the open point and the fault do not trip since no over current flows through these cable sections. The repair crew examines each FCI until they locate the last tripped FCI and the first untripped FCI; the failed cable section is between these two FCIs.
The trip indication takes different forms. More primitive devices require the repair crew to open each transformer enclosure and examine whether a mechanically operated indicator (operated by high current) shows an over current. The trouble crew must also mechanically reset these devices. More sophisticated devices have external indicators such as flashing lights and buzzers that beep. Some FCIs are radio-controlled. The more modern FCIs automatically reset when voltage returns to the circuit. A large number of these devices are needed to monitor a large distribution system. The problem with traditional FCIs is that they are expensive to install, time consuming to operate and somewhat unreliable.
Alternate technologies have been developed that use radar methods to locate faults. In these cases traveling waves are used to measure a time delay to the fault. Two primary methods are in use. The first method uses the transient waveform generated by the fault breakdown. In this case, the fault transient reflects back and forth between the fault and the end of the cable where the measurement is made. The round trip time delay is measured and interpreted as a distance to the fault. In the second case, a pulse signal is transmitted along the cable and is reflected from the fault. In this case, the pulse must be transmitted along the cable while the arc at the fault is ignited. The pulse is reflected from the arc and the round trip is interpreted as a distance to the fault. Both of these technologies are well known and have been in use for decades.
U.S. Pat. No. 5,206,595 to Wiggins et al. relies on the fault transient generated during breakdown. This method places a detector at the open circuit end of the cable and relies upon timing information from arriving signals that start and stop timing circuits. When the breakdown occurs, the transient travels to the end of the cable and starts a timer. It reflects from the end of the cable, travels back to the fault where it is again reflected and travels back to the open end of the cable. Upon arrival at the open end of the cable the timer is stopped. This round trip time is interpreted as a distance to the fault using a velocity which must be known.
The approach in U.S. Pat. No. 5,206,595 may not detect all faults and may erroneously detect non-fault conditions as faults. It is well known that URD cable systems are not homogeneous and have many discontinuities that give rise to interfering reflections. These interfering reflections can cause false stop signals in the timing circuit. Furthermore, the amplitude of the transient depends upon the voltage on the cable at the time of failure. Since this is not known prior to failure, this system must use a preset threshold in the comparator circuits to start and stop the timer. To reliably detect transients the threshold must be set low enough to trigger off the smallest anticipated transient. Having the threshold set low enough to ensure adequate sensitivity further aggravates the susceptibility to false stop triggers.
Another possible source of errors in a system of the type disclosed in U.S. Pat. No. 5,206,595 relates to the position of the sensor. The sensor is positioned at the end of the cable system. If a fault occurs close to the sensor it can not measure the round trip time to the fault unless extremely fast timers are used. To accurately measure the fault location very wide band electronics must be used. Using wide band electronics has the drawback that the circuitry is more susceptible to interfering reflections since they usually have higher frequency content than the transients of interest. Furthermore, this approach has no means to verify whether the transient was indeed from a true fault or some other transient event.
In U.S. Pat. No. 4,500,834 to Ko et al. an apparatus is described that places a detector at the end of the protection zone on a transmission line with protective relays. The fault detector is part of the protective relay located at the end of the zone and simply provides an indication of whether the fault is within the protected zone. The detector measures the time between successive reflections to determine whether the time delay is less than a predetermined value. If the time delay is less than the prescribed value the fault is in the protected zone otherwise it is not. The device does not provide a distance to the fault. Also, in a power transmission line system there are multiple protected zones and as such there will need to be multiple protective relays with this capability.
In U.S. Pat. No. 4,766,549 to Schweitzer, III et al. a system is described that applies to overhead transmission line power systems. The method uses a timing means to measure the time between successive reflections arriving at the end of the transmission line. This approach may have problems similar to those in U.S. Pat. No. 5,206,595. Because, however, the signals are filtered heavily prior to sampling at a low frequency some of the problems associated with wideband signals are mitigated. Furthermore, the system described in U.S. Pat. No. 5,206,595 may not be suitable for use in URD cable systems. When a fault occurs on an underground cable the fault transient is trapped between the fault and cable end. The fault has an extremely low impedance and no traveling wave energy can travel past it. In contrast, when a fault occurs on an overhead line the fault transient can travel across the fault since it has a much higher impedance. This difference in the transmission line structure makes the fault signatures and their interpretation much different.
The present invention is embodied in a system which locates the position of faults in URD cable systems. The cable system is continuously monitored and when a fault occurs its signature is recorded. A model of the fault signature is calculated using the recorded signature and the initial conditions of the cable system prior to the fault. The parameters of the model give the location of the fault which are used to sectionalize the cable system to isolate the failed cable section and return power to the customers.
URD cable systems typically are arranged in two halves, each half comprising a circuit length from the overhead feed point to an open point with transformers and cable sections located along the circuit length. According to one aspect of the invention, Each half loop uses one FDI. The FDI device monitors the circuit using an antenna which receives radio-frequency signals emitted by line faults and records the signatures of these faults. When the xe2x80x9ctrouble crewxe2x80x9d arrives to repair the fault, the recorded signature is downloaded to a portable PC via a radio, infra-red or other data communications link. The downloaded data is analyzed and the crew is directed to the precise fault location.